Selective call receivers, such as pagers, are typically small, portable electronic devices powered by batteries that receive selective call signals and demodulate and decode the signals to derive selective call messages addressed to the selective call receiver. The selective call receiver circuitry is designed to include low energy consuming components. Yet, with the desired performance characteristics, the components of the radio frequency (RF) receiver circuit are typically high energy consuming components. Accordingly, it is desirable to selectively energize and de-energize the receiver circuitry. This practice is called battery saving. Reliable battery saving methods energize the receiver circuitry during reception of appropriate portions of the selective call signals to assure that selective call messages addressed to the selective call receiver are received.
Selective call signals typically utilize a binary frequency shift keying (FSK) asynchronous modulation signaling protocol, e.g., the POCSAG signaling protocol originally proposed by British Telecom (POCSAG is an anagram for the Post Office Code Standardization Advisory Group). Conventional asynchronous protocols comprise a lengthy preamble, followed by a synchronization code word (sync code). The sync code contains a predetermined binary sequence used for frame synchronization. After detection of the preamble and the sync code, the protocol operates as a synchronous protocol comprising multiple words of information including address words and data words with the sync code repeated at regular intervals to maintain synchronous communication until the signal is dropped. The selective call receiver, while in synchronous communication, energizes and de-energizes the receiver circuitry in accordance with conventional battery saving routines.
Since detection of the preamble and the sync code following is crucial for converting from asynchronous operation to synchronous operation, the receiver circuitry typically is energized for long periods of time to search for the preamble and, once the preamble has been detected, the receiver circuitry remains energized until the sync code is detected.
Thus, what is needed is a reliable battery saving method and apparatus which can provide additional battery saving during asynchronous selective call receiver operation.